Best way to deadhead zinnias based on botany

maineman(z5a ME)September 6, 2004

Hi all,

I like to grow zinnias and sometimes to breed them. However, for just ornamental uses, I realize that deadheading them is absolutely necessary in order to keep the plants growing and branching and healthy and re-blooming.

My question is, based on sound botanical principles, what is the best way to deadhead a zinnia? In other words, how far down on the stem should you go to make the cut? If you go all the way down to a primary junction where side branches occur, you remove a lot of perfectly healthy leaves. Which might or might not be a good thing to do.

If the leaves on this stem could and would send their photosynthesized sugars over to nourish other "competing" stems, then leaving a bunch of those leaves could be a good idea. But if those sugars only go "up stem" to nourish the flower you are just now removing, then there would be no advantage to the zinnia plant to retain those leaves because they would just compete for sun and root support with leaves on other stems that haven't flowered yet.

I want to better understand how a zinnia plant "works." When removing a fading flower in the deadheading process, how much pruning of the zinnia plant is beneficial? If you see a bud, you should probably retain it because it can grow out to produce another bloom. If I knew in detail how a zinnia plant works, I would know whether the leaves above or upstem from a bud support and nourish that bud or whether they actually compete with it.

I know that rose growers have a good understanding of how to combine deadheading and pruning in a way that is beneficial to the development of the rose bush. I am seeking similar enlightenment and comprehension with regard to zinnia culture.

This summer I have used several zinnia deadheading strategies. One strategy is to remove the stem all the way back to the first side branches, leaving a little stem above the junction for "die back". This strategy removes a maximum amount of zinnia leaves along with the bloom, and tends to "open up" the plant to sun and air circulation.

A less severe strategy is to remove the stem and two or three leaves along with it, but leaving some of the lower leaves on the stem in case a lateral stem might develop at the base of some of those leaves. (In many cases, if not all cases, those future laterals never appear.)

The least severe strategy is to remove just the flowerhead down to the first leaf below it, and retain all leaves on the branch of that flowerhead. That tends to look tacky because the points of surgery tend to remain visible.

But, on the assumption that botany "knows" how a zinnia plant works, how does botany suggest that deadheading and associated pruning on a zinnia plant should be done?


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The reason you have specific rules with roses is that you want to 'train' the bush to grow in a specific manner, e.g. without canes crossing, making sure the stems are thick enough to support new growth etc.

With zinnias you hardly have these considerations. Generally, if you deadhead very low, you get a lower growing plant. if you deadhead high, you get a taller plant (duh !!). The removal of leaf tissue preventing photosynthesis is not really a great concern for zinnias. They are very active growers, and if you decide to cut a zinnia back, it will branch at this particular spot, grow new stems and then flower. If you deadhead higher, the branching occurs at this higher level.

Of course, you can not sheer a zinnia to the ground repeatedly and expect flowers. However, you really have to be unluckly to cut a zinnia back to the point that it will not produce a good result.

With zinnias you can cut them back to nearest set of leaves, or you can cut them further down based on your desire. Zinnias are pretty forgiving in this respect.

    Bookmark   September 7, 2004 at 5:03PM
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Sorry forgot to mention,

if you cut a zinnia back to a low point, and then repeatly pinch out the new growing points after a few leaves develop, you can create a pretty bushy low growing bush, similar to a florist mum. However, the flowers become very small and uninteresting. You are probably better off concentrating flowering on a few isolated stems.. this particularly so for the larger flowered varities.


    Bookmark   September 7, 2004 at 5:05PM
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maineman(z5a ME)


"If you cut a zinnia back to a low point, and then repeatly pinch out the new growing points after a few leaves develop, you can create a pretty bushy low growing bush, similar to a florist mum."

I have done that on a few occasions and, yes, you are right about that. It is an interesting effect. The flowers are smaller, but the plant habit is so bushy and nice that it is almost worth it.

With respect to the high pruning or low pruning, I still wonder whether the sugars produced by a zinnia leaf go exclusively up-stem.


    Bookmark   September 8, 2004 at 1:48AM
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In the leaves, plants produce a carbohydrate called glucose (a sugar molecule) which is transported to other parts of the plant for energy. This carbohydrate is produced by photosynthesis.

The vascular system of plants consist of two parallel "arteries" the xylem and phloem.

In the xylem, water and dissolved nutrients are moved upwards (from the roots). This is a one-directional movement powered by a pumping action. The evaporation of water in the leaves creates a vacuum which helps pull water (and dissolved fertilizer nutrients) up the vascular pathways.

In the pholem, which is responsible for transporting glucose (sugars), the direction is not limited to one direction. The movement is from sugar producing tissues (sugar sources) to sugar consuming tissues (sugar sinks). Sugar sinks are consuming entities, such as fruit, flowers, roots, etc whereas sources are tissues which produce the glucose, or which is responsible for breaking starch down into glucose.

The transport inside the phloem is done by another force called hydrostatic pressure. In the phloem by the leaves water is mixed with high concentrations of glucose (sweet water) creating relatively high pressure (tugor). In the roots, the sugar is removed/consumed which causes water concentration in the phloem to drop. The result is that you have a high pressure at the production site and a low pressure at the consumption site. I.e. you have set up the requirements for a pump to work. The flow will be from high sugar concentration (low water concentration) to low sugar concentration (high water concentration). The reason this takes place is that water though osmosis will either flow into the tube or out of the tube to match the sugar flow (rules of diffusion).

Therefore, to answer your question:
By pruning off relatively large parts of the plant you are removing tissue which would otherwise be used to produce sugars (photosyntehsis potential), but sugars do not by rule move 'upstem'. Quite on the contrary. Most sugar is produced 'high on the plant' (in the leaves) and is often transported to the roots, although quite a bit also end up in flowers and thus, yes, some sugar does move 'up-stem'.

But you are not preventing this movement from taking place just because you prune high or low. These two issues are not connected.

If you want to know what happens when you prune a plant, take a look at the below article.

Here is a link that might be useful: Cutting and Pinching - article

    Bookmark   September 9, 2004 at 2:25PM
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maineman(z5a ME)


Thanks for your excellent explanation and the link to your equally excellent article. That is exactly the kind of information I was looking for, and it reflects well on this Botany forum, as well as Suite 101, as sources of authoritative information.

I didn't have a botany course in my formal schooling, but you have helped me to fill a part of that hole in my education. I view applied botany as an important component of the science of gardening, a subject area that I need to learn more about.


    Bookmark   September 9, 2004 at 5:42PM
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do you broadcst your seeds as a direct sow and if so what kind of soil are you braodcasting into?

The reason I ask is that in my experince, if I use plants from 6 packs verses self sown direct in a rich home brew of compost...I have less up keep for my zinnias, they branch better and overall look more helathy than the transplanted types. It may have to do with our shorter growing season in Maine, too. (amount of day light, temps, climate)

    Bookmark   September 14, 2004 at 12:01AM
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